Ammonite

The name “ammonite”, from which the scientific term derived, inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams’ horns. Pliny the Elder (d. 79 AD near Pompeii) called fossils of these animals ammonis cornua (“horns of Ammon”) because the Egyptian god Ammon (Amun) typically depicted wearing ram’s horns.

Ammonoids are an extinct group of marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs are more closely related to living coleoids than they are to shelled nautiloids. The earliest ammonites appear during the Devonian, and the last species died out during the Cretaceous–Paleogene extinction event.

Ammonite are excellent index fossil. It is often possible to link the rock layer in which a particular species or genus found to specific geologic time periods. Their fossil shells usually take the form of planispirals, although there some helically spiraled and nonspiraled forms.